1-Open the picture you want to work on. On this tutorial we are going to use this one:

2-Select the eliptical marquee tool

3-And draw a shape around the area you want it to be censored. We are going to censore the chest.

4-Go to FILTER>PIXELATE>MOSAIC and choose something between 5 and 10.

5-And it's done, simple and easy but cool :)

Kal El